Pistons for medical hollow bodies, in particular syringes or carpules, are known. They typically have a substantially cylindrical shape and can be introduced in a likewise substantially cylindrical interior of a medical hollow body. On an outer surface shell, such a piston has at least one sliding surface with which said piston rests against an inner surface shell of an interior of the hollow body. Preferably, the piston comprises a material, at least in the area of this sliding surface, which is more elastic than the material of which the medical hollow body is made. Moreover, the outer diameter of the piston is preferably slightly larger than the inner diameter of the medical hollow body. If the piston is introduced into the medical hollow body in such a manner that the longitudinal axes of the piston and the hollow body are aligned with each other, this results in an at least slight compression of the piston material in the area of the at least one sliding surface so that a sealing effect is established here. Thus, the piston sealingly closes the interior of the hollow body. At the same time, a displacement of the piston within the hollow body is possible, wherein the piston slides with its at least one sliding surface on the inner surface shell of the hollow body. This sliding movement of the piston can be used to optionally expel or draw in a medium, for example, a pharmaceutical substance or a sample, through an opening of the hollow body which—viewed in the axial direction of the hollow body—is located opposite to the piston.
Known pistons for medical hollow bodies comprise materials which, in connection with the material of which the hollow body is made, have high coefficients of static and sliding friction in the area of the at least one sliding surface of the piston when the latter rests against the hollow body so that a displacement of the piston relative to the hollow body with an acceptable exertion of force is not possible without further measures. Depending on the elasticity of the piston, the latter can deform when axial forces are introduced, which results in an increase of the static friction in the area of the at least one sliding surface. An increase of the axial forces results in that the friction forces counteracting a displacement of the piston are intensified so that finally a blocking of the piston occurs and therefore the piston can no longer be displaced. In the case of a further increase of the axial forces, this can result in a destruction of the piston.
In order to allow a displacement of the piston with acceptable force exertion while avoiding a blocking, it is known to coat the inner surface shell with a lubricant so that the piston can slide during its displacement in the hollow body with its at least one sliding surface on a lubricating film formed by the lubricant. Typically, silicone, silicone oil, and/or silicone oil emulsions are used as a lubricant.
In order to allow a displacement of the piston along the entire axial extension of the interior of the hollow body, the entire inner surface shell of the latter has to be coated with the lubricant. This is usually carried out prior to introducing the piston into the hollow body and in particular prior to filling the hollow body with a pharmaceutical substance. Hereby, the pharmaceutical substance is in contact with the lubricant during storage of the prefilled hollow body. The disadvantage of this is that in particular in the case of biotechnologically produced, sensitive pharmaceutical substances, interactions with the lubricant can take place. In particular, it is known that silicone oil can result in instability of active ingredients which comprise proteins and/or peptides. For example, aggregate formation or precipitation can take place. Also, the potentially forming aggregates are suspected to trigger a series of undesirable immune reactions. The coating of the interior of a medical hollow body which comprises a substance intended to be injected into a patient thus can result not only in a reduction of the efficacy, but can in fact have negative consequences for the health of the patient to whom the injection is administered.